Presently, there is a great demand for reduced semiconductor device dimensions to provide an increased density of devices, on the semiconductor chip, that are faster and consume less power. The scaling of the devices in the lateral dimension requires vertical scaling as well so as to achieve adequate device performance. This vertical scaling requires the thickness of the gate dielectric to be reduced so as to provide the required device performance. However, thinning of the gate dielectric provides a smaller barrier to dopant diffusion from the gate structure, through the dielectric, and into the substrate.
In order to use lower voltage supplies, reduce power consumption, and maximize transistor performance, boron doped gates are preferred for PMOS devices due to better short-channel control than phosphorous doped gates. Phosphorous doped gates result in buried channel PMOS devices while boron doped gates yield a surface channel device.
While boron doping of gate structures solves some problems it causes others, because boron is a rapid diffuser in polysilicon ("poly") and oxide. More specifically, because of the thermal cycles required in today's processing along with the continued down-scaling of the gate dielectrics, the diffusion of boron through the poly gate structure and the thin gate dielectric may cause damage to the underlying channel region along with degrading the dielectric reliability and reducing the control over the threshold voltage of the device. Hence, with thinner gate oxides and shorter channel lengths, any boron penetration into the channel region can cause loss of control over the threshold voltage of the device, and, in the worse case, cause the channel region to be short-circuited.
One attempt at solving this problem involves incorporating nitrogen into the polysilicon gate. However, this method has problems. First, nitrogen is a donor in silicon and, therefore, it may create an n-type layer at the polysilicon/ gate insulator interface. Hence, this would counteract the benefit of having boron in the polysilicon gate structure. Second, the common source for nitrogen doping is NH.sub.3, which introduces H and OH traps into the dielectric which can reduce the dielectric's charge-to-breakdown and degrade hot carrier stability.
It is, therefore, an object of the instant invention to provide a gate structure which will properly retard boron penetration into the substrate through the gate insulator without degrading the performance of the device. More generally, it is an object of the instant invention to provide a gate structure which will inhibit the dopant used to make the gate structure more conductive from penetrating into the channel region.